I don't think a voltage regulator is the right application, as DVDdoug already told you. It's a shame that you didn't listen. Voltage regulators are designed to POWER something. They are never used to regulate the voltage on a data line. Good luck making that work.
Actually I am not using it to trigger any arduino pin.. I am using a 8 PIN PIC with a very tiny PCB.. Space is a real issue. So if I am using a SOT-89 7805 I do not require any other component.
I definitely agree that using an Optoisolator is the best option, but it requires many other components too and will occupy lil larger space than what the SOT-89 7805 will occupy.